Complex behavior patterns traditionally have been assumed to be associated with complex nervous organization. Yet little is known of the behavioral capacities of elementary nervous systems. The goal of this study is to elucidate the physiological bases of behavior in the hydroids, Hydra and Tubularia. The structure of the nervous system in these animals is simple, that of a nerve net. Recent evidence indicates that components of hydroid behavior are controlled by non-nervous mechanisms. Several questions are posed in this study. What degree of behavioral complexity can be coordinated by nerve net functions and non-nervous activity? Are long-term processes of habituation and dishabituation present? Is the system capable of associative learning? What are the roles of nerve cells and epithelial conducting systems in the control of behavior? What are the properties of individual pacemakers and how are they integrated? A range of behavioral and electrophysiological techniques are employed. Quantitative analysis of pulse interval measurements and computer simulation are used to construct modes describing processes in this behavioral system. Such studies investigate processes coordinating behavior in hydroids, giving us insight into properties of elementary nervous systems.